Cryogenic air separation process and apparatus producing elevated pressure nitrogen by pumped liquid nitrogen

Abstract: Air is separated in a cryogenic air separation process in which (i) at least a portion of a nitrogen-rich liquid (410) from a column system having high and low pressure columns (5,6) is boosted in pressure (13) before being vaporized (2) and delivered as a product (400); (ii) at least a portion (120) of the feed air (100) is at least partially condensed in indirect heat exchange (2) with the boosted pressure, nitrogen-rich stream; and (iii) a portion of the liquid nitrogen condensed from the vapor nitrogen from the top of the higher pressure column (5) is returned to the higher pressure column (5) as reflux with any remaining portion (40) being removed from the column system.

Description: The process of the present invention relates to a process and an apparatus for the production of pressurized oxygen and nitrogen products by the cryogenic distillation of air. There are numerous situations for which both pressurized oxygen and pressurized nitrogen are required. Since equipment cost and power cost are the important aspects of the cost of production, an objective of the present invention is to reduce the equipment or power cost, or both, for a process to produce both pressurized oxygen and nitrogen products. US-A-5148680 (published 22nd September 1992; corresponding to EP-A-0464630 published 8th January 1992) discloses a process and apparatus for cryogenic air separation using a double column distillation system in which liquid oxygen produced in the low pressure column is pressurized and then used to condense a feed air portion. The condensed feed air portion and all other feed air portions are fed to the high pressure column. Liquified nitrogen product from the high pressure column also is pressurized and then used to condense the feed air portion. Top reflux to the low pressure column is provided by an impure liquid nitrogen stream from an intermediate location of the high pressure column or by liquified nitrogen product from the high pressure column. The preambles of the independent claims are based on US-A-5148680. GB-A-2251931 (published 22nd July 1992) also discloses a process and apparatus for cryogenic air separation using a double column distillation system in which liquid oxygen produced in the low pressure column is pressurized and then used to condense a pressurized feed air portion. At least part of the condensed feed air portion is fed to an intermediate location of the low pressure column. Top reflux to the low pressure column is provided by liquified nitrogen product from the high pressure column. In the only exemplified embodiment (Figure 2) of GB-A-2251931, all the nitrogen product from the high pressure column is fed to the low pressure column as top reflux. EP-A-0504029 (published 16th September 1992) also discloses a process and apparatus for cryogenic air separation using a double column distillation system in which liquid oxygen produced in the low pressure column is pressurized and then used to condense a pressurized feed air portion. At least part of the condensed feed air portion is fed to an intermediate location of the low pressure column. Top reflux to the low pressure column is provided by liquid nitrogen product from the high pressure column. In one exemplified embodiment (Figure 1) of EP-A-0504029, an impure liquid nitrogen stream from an intermediate location of the high pressure column is fed to the low pressure column at a location between the condensed air feed and the top reflux. In this embodiment, part of the liquid nitrogen product from the high pressure column is pressurized and, optionally. also used to condense the high pressure feed air portion. The present invention provides a process for the separation of a compressed feed air stream to produce elevated pressure oxygen and nitrogen gases comprising: (a) using a double column system with a lower pressure column and a higher pressure column; (b) feeding at least a portion of the compressed and cooled feed air to the higher pressure column; (c) separating the portion of the feed air from step (b) into a nitrogen vapor and an oxygen-enriched liquid in the higher pressure column; (d) feeding the oxygen-enriched liquid from the bottom of the higher pressure column to an intermediate point in the lower pressure column; (e) condensing at least a portion of a nitrogen-rich vapor from the higher pressure column thereby producing a liquid nitrogen stream. returning at least a portion of the liquid nitrogen stream to the top of the higher pressure column; and removing any remaining portion of the liquid nitrogen from the double column system; (f) increasing the pressure of a nitrogen-rich liquid which is removed from a location of the higher pressure column; (g) cooling and at least partially condensing a portion of the feed air by indirect heat exchange with the elevated pressure nitrogen-rich stream of step (f); and (h) removing an oxygen stream and a vapor stream containing at least 80% nitrogen from the lower pressure column, characterized in that (i) condensed feed air from step (g) is fed to the top of the lower pressure column and in that (ii) top reflux to the lower pressure column is provided by said condensed feed air and optionally by an impure liquid nitrogen stream withdrawn from an intermediate location of the higher pressure column. The present invention also relates to the process described above wherein the oxygen stream of step (h) is a liquid and the pressure of the liquid oxygen stream is boosted to a higher pressure and vaporized by indirect heat exchange with a second portion of feed air thereby at least partially condensing that portion of feed air. In another aspect. the present invention provides an apparatus for carrying out the process of the invention, said apparatus comprising: (i) a double column system with a lower pressure column and a higner pressure column, (ii) conduit means for feeding at least a portion of the compressed and cooled feed air to the higher pressure column for separating into a nitrogen vapor and an oxygen-enriched liquid; (iii) conduit means for feeding the oxygen-enriched liquid from the bottom of the higher pressure column to an intermediate point in the lower pressure column; (iv) a condenser for condensing at least a portion of a nitrogen-rich vapor from the higher pressure column to produce a liquid nitrogen stream; (v) conduit means for returning a portion of said liquid nitrogen stream to the top of the higher pressure column; (vi) conduit means for removing any remaining portion of the liquid nitrogen from the double column system; (vii) a pump for increasing the pressure of a nitrogen-rich liquid which is removed from a location of the higher pressure column; (viii) heat exchange means for cooling and at least partially condensing a portion of the feed air by indirect heat exchange with said elevated pressure nitrogen-rich liquid; and (ix) conduit means for removing an oxygen stream and a vapor stream containing at least 80% nitrogen from the lower pressure column, characterized in that conduit means is provided to feed condensed feed air from said heat exchange means to the top of the lower pressure column and optionally, conduit means is provided to feed an impure liquid nitrogen stream from an intermediate location of the higher pressure column to the top of the lower pressure column. The following is a description of presently preferred embodiments of the invention. In the drawings: Figures 1 and 2 are schematic diagrams of three embodiments of the process of the present invention. The process of the present invention has three important features: (1) at least a portion of a nitrogen-rich liquid from the column system is boosted in pressure before being vaporized and delivered as a product; (2) at least a portion of the feed air is at least partially condensed in indirect heat exchange with the boosted pressure. nitrogen-rich stream: and (3) at least a portion of the liquid nitrogen condensed from the vapor nitrogen from the top of the higher pressure column is returned to the higher pressure column as reflux with any remaining portion being removed from the column system. In the preferred mode, the portion of liquid nitrogen leaving the column system in step (3) provides the nitrogen-rich liquid in step (1). When the nitrogen-rich liquid in step (1), is withdrawn from a different location of the column system, the removed portion of liquid nitrogen in step (3) can be zero. In the most preferred mode, a portion of liquid oxygen from the column system is pumped to an elevated pressure and is also vaporized by heat exchange with a portion of the feed air stream which is at least partially condensed. This will coproduce an elevated pressure oxygen product stream. The process of the present invention can be best understood witn reference to several specific embodiments thereof. Figure 1 shows one embodiment of the present invention. With reference to Figure 1, feed air, line 100, which is compressed and free of contaminants, is first split into two substreams, lines 102 and 120. The first substream, line 102, is cooled in heat exchanger 1 to a cryogenic temperature and mixed with an expander effluent, line 108, to form the higher pressure column feed, line 110, which is then fed to higher pressure column 5. The other substream, line 120, is further boosted in pressure to a pressure, eg above 600 psia (4 1 MPa), higher than that of the high pressure column 5, by compressor 14, then, line 122, cooled and further split into two parts, lines 140 and 124. The first part, line 140, is cooled in heat exchanger 2 to an intermediate temperature end then sentropically expanded in expander 12. The expander effluent, line 108, is mixed with the first portion of cooled air, line 106, to form the higher pressure column feed, line 110. The second part, line 124, is yet further compressed by compressor 11 which is mechanically linked to expander 12. Additionally or alternatively, expander 12 can be coupled with an electric generator. The further compressed second part is then aftercooled, further cooled in heat exchanger 2. to a temperature below -220°F (-140°C), preferably below -250°F (-155°C) (thus, becoming a dense fluid), line 152, and split into two portions, lines 157 and 158. The first portion of this dense fluid, line 157, can be fed to higher pressure column 5 at an intermediate location. The remaining portion, line 158, is further subcooled in subcooler 3. This subcooled portion, line 162, is then fed to the top of lower pressure column 6 as reflux. The feed to higher pressure column 5, lines 110 and 157, is distilled and separated into a nitrogen vapor stream and oxygen-enrich bottoms liquid. The vapor nitrogen is condensed in a reboiler/condenser located in the bottom of lower pressure column 6. A portion of this liquid nitrogen is returned to higher pressure column 5 as reflux. The remaining portion, line 40, is split into the product liquid nitrogen, line 600, and the liquid nitrogen to be boosted in pressure, line 410. The liquid nitrogen to be boosted in pressure, line 410, is then pumped to a higher pressure by pump 13 and heated and vaporized in heat exchanger 2 resulting in an elevated pressure and close to ambient temperature gaseous nitrogen product, line 400. The oxygen-enriched bottoms liquid from higher pressure column 5, line 10, is fed into lower pressure column 6 at an intermediate position. This stream and the liquid air fed to the top of lower pressure column 6, line 162, are distilled in lower pressure column 6 and separated into a liquid oxygen bottoms and a nitrogen-rich overhead containing at least 80% nitrogen. A portion of the liquid oxygen bottoms, line 20, is removed from the bottom of lower pressure column 6 and then split into a liquid oxygen product, line 700, and a portion that is vaporized and heated up to a temperature close to ambient in heat exchanger 1 and removed as gaseous oxygen product, line 200. The nitrogen-rich overhead is removed from the top of lower pressure column 6, line 30, is heated in subcooler 3 and split into two portions, lines 304 and 312. These two streams are then heated up in heat exchangers 1 and 2, respectively, to ambient temperatures before being vented or used for air cleaning adsorption bed regeneration, lines 300, 310. The embodiment shown in Figure 2 is similar to the one shown in Figure 1. The differences are described below. First, the second compressed feed air substream, line 124, is still further compressed and then split into two subparts, lines 144 and 126. The first subpart, line 126, is cooled in indirect heat exchange with the warming oxygen stream in heat exchanger 4. further split into two streams, lines 130 and 148, at an intermediate point of heat exchanger 4. The first stream, line 130, is further cooled to a temperature below the critical temperature of air by indirect heat exchange with warming oxygen in heat exchanger 4. The other subpart, line 144, is cooled in heat exchanger 2, combined with the stream, line 148, from heat exchanger 4 at an intermediate temperature and further cooled to a temperature below -220°F (-140°C), preferably below -250°F (-155°C). The higher pressure air streams that are cooled below -220°F (-140°C), lines 152 and 132, are then combined. Second, the liquid oxygen, line 20, from lower pressure column 6, is pumped to a higher pressure by pump 15 and then vaporized and heated to ambient temperature in heat exchanger 4. A portion of the condensed liquid nitrogen, line 40, is warmed against feed air 102 in heat exchanger 1 before removal as product, line 800. As an option, an impure liquid nitrogen stream, line 42, is withdrawn from an intermediate location of the higher pressure column, subcooled in the cold section of subcooler 3 and fed along with the subcooled liquid air, line 162, to the top of lower pressure column 6, line 164. Results of a simulation using the embodiment of Figure 2 are summarized in the following Table. The purities of products oxygen (stream 200) and nitrogen (streams 400 and 600) are 98% O 2 and 6 vppm O 2 , respectively. Stream Number 100 122 140 152 158 Temperature: °F 104.0 104.0 104.0 -276.9 -267.9 (°C) (40.0) (40.0) (40.0) (-171.6) (-166.6) Pressure: psia 85.5 750 750 1150 1028.3 (MPa) (0.590) (5.171) (5.171) (7.929) (7.090) Flow: lbmol/h 100.0 73.0 33.3 31.0 31.0 (kgmol/h) (45.35) (33.10) (15.10) (14.05) (14.05) Stream Number 200 300 310 400 Temperature: °F 73.8 88.8 83.8 88.8 °C (23.2) (31.55) (28.8) (31.55) Pressure: psia 1450 16.2 15 1133.5 (MPa) (9.998) (0.112) (0.103) (7.815) Flow: lbmol/h 17.3 33.7 23.7 20.3 (kgmol/h) (7.85) (15.30) (10.75) (9.20) 20 40 600 42 800 Temperature: °F -291.0 -288.2 -288.2 -288.2 83.8 °C (-179.4) (-177.9) (-177.9) (-177.9) (28.8) Pressure: psia 21.2 79.7 79.7 79.7 85.5 (MPa) (0.146) (0.550) (0.550) (0.550) (0.590) Flow: lbmol/h 17.3 27.0 .1 1.8 4.9 (kgmol/h) (7.85) (12.25) (0.05) (0.80) (2.20) An unexpected benefit of the present invention, since a fraction of the partially condensed feed air portion is fed to the top of the lower pressure column as impure reflux and where product pressures are high, is that the lower oxygen recovery resulting from having no nitrogen reflux in the lower pressure column does not result in an overall energy penalty or a capital penalty. The process of the present invention is particularly advantageous when both oxygen and nitrogen are required at very high pressures. The present invention has been described with reference to several specific embodiments thereof. These embodiments should not be seen as a limitation on the present invention.

Claims: A process for the separation of a compressed feed air stream (100) to produce elevated pressure oxygen and nitrogen gases comprising: (a) using a double column system with a lower pressure column (6) and a higher pressure column (5) ; (b) feeding at least a portion (110) of the compressed and cooled feed air to the higher pressure column (5); (c) separating the portion of the feed air from step (b) into a nitrogen vapor and an oxygen-enriched liquid (10) in the higher pressure column (5); (d) feeding the oxygen-enriched liquid (10) from the bottom of the higher pressure column (5) to an intermediate point in the lower pressure column (6) ; (e) condensing at least a portion of a nitrogen-rich vapor from the higher pressure column (5) thereby producing a liquid nitrogen stream; returning at least a portion of the liquid nitrogen stream to the top of 'the higher pressure column (5); and removing any remaining portion (40) of the liquid nitrogen from the double column system; (f) increasing the pressure (13) of a nitrogen-rich liquid (40) which is removed from a location of the higher pressure column (5); (g) cooling and at least partially condensing (2) a portion (144) of the feed air by indirect heat exchange with the elevated pressure nitrogen-rich stream of step (f); and (h) removing an oxygen stream (20) and a vapor stream (30) containing at least 80% nitrogen from the lower pressure column (6), characterized in that (i) condensed feed air (158) from step (g) is fed to the top of the lower pressure column (6) and in that (ii) top reflux to the lower pressure column (6) is provided by said condensed feed air (158) and, optionally, an impure liquid nitrogen stream (42) withdrawn from an intermediate location of the higher pressure column (5). A process as claimed in Claim 1, wherein the nitrogen-rich liquid from the column system of step (f) is a portion (410) of liquid nitrogen (40) removed from the column system in step (e). A process as claimed in Claim 1, wherein the nitrogen-rich liquid of step (f) is taken from an intermediate position of the higher pressure column (5). A process as claimed in Claim 3, where all of the liquid nitrogen produced in step (e) is returned as reflux to the high pressure column (5). A process as claimed in any one of the preceding claims, wherein the oxygen stream (20) of step (h) is a liquid and the pressure of the liquid oxygen stream is boosted (15) to a higher pressure and vaporized by indirect heat exchange (4) with a second portion (126) of feed air thereby at least partially condensing that portion of feed air. A process as claimed in Claim 5, wherein the majority of the combined amount of the condensed feed air portions (132 & 152) is fed (158, 162) to the top of the lower pressure column (6). A process as claimed in any one of the preceding claims, wherein the feed air (140,144) that is at least partially condensed is compressed to a pressure higher than 4.1 MPa (600 psia) before being cooled to a temperature below -140°C (-220°F). A process as claimed in Claim 7, wherein the at least partially condensed air (152) is a dense fluid. A process as claimed in any one of the preceding claims, wherein top reflux to the lower pressure column (6) is entirely provided by the condensed feed air (158). A process as claimed in any one of Claims 1 to 8, wherein top reflux to the lower pressure column (6) is provided in part by the condensed feed air (158) and in part by the impure liquid nitrogen stream (42). A process as claimed in any one of the preceding claims, wherein a high pressure air stream (140) is expanded from a higher pressure to a lower pressure through isentropic expansion (12). A process as claimed in Claim 11, wherein the expander (12) for isentropic expansion of the high pressure air stream (140) is coupled to a compressor (11). A process as claimed in Claim 12, wherein the compressor (11) coupled with the expander (12) is used to compress an air stream (124) with a pressure higher than that of the higher pressure column (5). A process as claimed in Claim 11, wherein the expander (12) for isentropic expansion of the high pressure air stream (140) is coupled with an electric generator. A process as claimed in any one of the preceding claims, wherein a gaseous oxygen stream is produced directly from the bottom of the lower pressure column (6). A process as claimed in any one of the preceding claims, wherein a nitrogen rich gas stream is produced directly from the higher pressure column (5). An apparatus for separating a compressed feed air stream by a process as claimed in Claim 1, said apparatus comprising: (i) a double column system with a lower pressure column (6) and a higher pressure column (5); (ii) conduit means (110) for feeding at least a portion of the compressed and cooled feed air to the higher pressure column (5) for separating into a nitrogen vapor and an oxygen-enriched liquid; (iii) conduit means (10) for feeding the oxygen-enriched liquid from the bottom of the higher pressure column (5) to an intermediate point in the lower pressure column (6); (iv) a condenser for condensing at least a portion of a nitrogen-rich vapor from the higher pressure column (5) to produce a liquid nitrogen stream; (v) conduit means for returning a portion of said liquid nitrogen stream to the top of the higher pressure column (5); (vi) conduit means (40,400,410,600) for removing any remaining portion of the liquid nitrogen from the double column system; (vii) a pump (13) for increasing the pressure of a nitrogen-rich liquid (410) which is removed from a location of the higher pressure column (5) ; (viii) heat exchange means (2) for cooling and at least partially condensing a portion of the feed air by indirect heat exchange with said elevated pressure nitrogen-rich liquid; and (ix) conduit means (20,30) for removing an oxygen stream and a vapor stream containing at least 80% nitrogen from the lower pressure column, characterized in that conduit means (158,162) is provided to feed condensed feed air from said heat exchange means (2) to the top of the lower pressure column (6) and, optionally, conduit means (42) is provided to feed an impure liquid nitrogen stream from an intermediate location of the higher pressure column (5) to the top of the lower pressure column (6). An apparatus as claimed in Claim 17, wherein conduit means (410) removing liquid nitrogen from the column system supplies a portion of the removed nitrogen to said pump (13) to provide the said nitrogen-rich liquid. An apparatus as claimed in Claim 17, wherein conduit means supplies the said nitrogen-rich liquid from an intermediate position of the higher pressure column (5) to said pump (13). An apparatus as claimed in any one of Claims 17 to 19, wherein said conduit means (20) for removing an oxygen stream from the lower pressure column (6) conveys liquid oxygen to a pump (15) to boost the pressure thereof and then to a heat exchanger (4) for indirect heat exchange with a second portion of feed air thereby at least partially condensing that portion of feed air. An apparatus as claimed in any one of Claims 17 to 20, including conduit means (42) conveying an impure liquid nitrogen stream from an intermediate location of the higher pressure column (5) to the top of the lower pressure column (6) as reflux.

Inventors: AGRAWAL RAKESH, DAWSON BRUCE KYLE, HOPKINS JEFFREY ALAN, XU JIANGUO, AGRAWAL, RAKESH, DAWSON, BRUCE KYLE, HOPKINS, JEFFREY ALAN, XU, JIANGUO
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1994
Main classification: F25J3
IPC codes: F25J3